[AMPS] Grid dissipation

Rich Measures measures@vc.net
Fri, 8 Aug 97 08:41:58 -0800

>> From:          Rich Measures <measures@vc.net>
>> Date:          Fri, 8 Aug 97 07:24:35 +0000
>> - There appears to be a lot of technoblather floating around about the 
>> extreme delicacy of the 8877's grid.  However, at the maximum ratings of 
>> 250MHz, 1A, and 4000V, over 40 rms amperes of RF current flows through 
>> the grid to chassis ground/common.  
>You really should not give false advice like this to people trying to 
>learn.  Twisted statements like you made above ...
Taking a closer look at the allegations of false and twisted:  

-  The maximum ratings are from the (revised 5-15-77) Eimac Tech. Data 
sheets for the 8877: 250MHz/1A/4000V.  According to the characteristic 
curves in these Tech. Data sheets, the minimum anode/plate to (grounded) 
grid V is roughly 300V.  With a 4000V anode supply, the resultant AC 
anode voltage is 4000 minus 300V = 3700V peak, or 2600Vrms.  
- According Eimac, the 8877 has about 10pF of C between the anode and the 
grid.  At 250MHz, 10 pF has 64 ohms of capacitive reactance.  The AC 
current through the grid = 2600V rms/64 ohms = 40.6A rms.  ....... Sure, 
40A is a fair amount of current.  However, the 8877 has over 100 
rectangular grid wires, so the current is only about 400mA per 
gold-plated grid wire.  Since gold is a fairly good conductor and gold 
does not begin to evaporate in a vacuum until its temp. exceeds 1800 
degrees F, why should there be a problem?


R. L. Measures, 805-386-3734, AG6K   

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